In these systems, referred to in the art as TPMS (Tire Pressure Monitoring System), the signals sent by the pressure sensors include data which allows them to be identified. If the wheels on a vehicle were never changed, simple initial learning of the association between the signals and the wheels would allow the position of the sensors in relation to the vehicle's chassis to be determined. This analysis is necessary to be able to locate defective tires.
However, as the wheels on a vehicle are from time to time re-positioned or swapped over, the problem of locating the wheel on which the sensor is mounted remains.
Some solutions have already been proposed. In particular, in patent application FR 0116368, the Applicant describes a tire pressure monitoring system where the rotation speeds calculated using the data supplied by the pressure sensors is compared with the data determined by fixed wheel rotation speed sensors with a known position. In particular, the latter relate to ABS (Anti-lock Brake System) sensors. The ABS system uses an individual sensor for each wheel giving its angular position in the form of pulses. These correspond to the distances that the wheel travels in the order of several centimeters.
It is known that the wheels on a vehicle do not all rotate at the same speed. In a straight line, for example, a difference in the tire pressures, or even in the degree of wear in the tire tread, leads to a difference in the wheel diameter and therefore the rotation speed. The vehicle's dynamics, the load distribution and the temperature are also factors which influence the rotation speed of the wheels in relation to each other.
When the trajectory is curved, the differences in rotation speeds due to the difference in the radiuses of curvature of each wheel's trajectory are added thereto.
These differences are utilized, therefore, to conduct this analysis. To this end, the difference is calculated between the speed of each fixed sensor and that speed which is calculated using the data supplied by the pressure sensors. In this way, each of the fixed sensors can be associated with the pressure sensor with the lowest calculated difference in speed. This system is advantageous in that it uses existing means and thus avoids additional expenditure. In addition, it has proved reliable as regards to the quality of the fixed sensors on the anti-lock brake system.
However, there are many instances where the speeds measured are all very similar. The convergence of the calculation is therefore rather slow and the system's reaction time is relatively long; up to 15 minutes have been counted. The pressure sensors are powered by batteries located inside the tires which, therefore, makes access difficult. It is therefore advisable to limit their energy consumption as much as possible. It would therefore be advantageous to limit the transmission time of the sensors.
In application FR 02 11 843, the Applicant, having set an objective of improving the wheel location means in a tire pressure monitoring system by using a means which reduced the transmission time of the sensors, proposed attaching to the pressure sensor means for determining the phase shift between two pressure signal transmissions performed at two points, T1 and T2. However, this solution did not fully satisfy either the efficiency requirements which it set out to achieve.
This is the reason why the Applicant is now proposing an improved version of this invention.